The present invention relates generally to a device and method for detection of specific chemical components in an environment containing many distinct chemical species. More particularly, the present invention relates to a chemical switch device comprising a film which irreversibly reacts upon exposure to specific chemical components in the environment under the conditions of measurement. The reactions can lead to large changes in the physical and chemical properties of the film which are measurable electrically, optically or by other methods. The term "conditions of measurement" is intended to mean any environmental conditions under which a reacted or unreacted state of the device can be determined. The chemical switches are "yes-no," intrinsically binary devices that can be miniaturized, mass produced and directly incorporated into digital electronic circuits.
Typically, electrical switches, such as fuses, are used to provide a break in an electrical circuit to prevent an electrical overload. Conventional irreversible switches are thermal fuses which fail by physical breakage of the electrical path due to resistive overheating. In the electrical embodiment of the present invention, an irreversible chemical switch can exhibit a large change in physical or chemical properties upon reaction of the switch material with a specific chemical component. The apparent physical or chemical change can manifest itself as a measurable increase or decrease of resistance due to an irreversible change in the switch material from a conductor or resistor to a resistor or conductor, respectively, upon reaction with a specific chemical component. The change is irreversible under the conditions of measurement, much like a conventional electrical fuse, except that the resistance of the irreversible chemical switch can either increase or decrease upon exposure to specific chemical components. Furthermore, other properties of the device, such as optical or thermal properties, can be used to monitor the extent of the irreversible reactions. The present invention also provides methods for detection of individual chemical components, e.g., hazardous gases, in an environment using an irreversible chemical switch.
The concept of an irreversible chemical switch, based upon irreversible chemical reactions, is believed to be heretofore unknown. In contradistinction to electrical fuses, which fail by thermal breakage of a conductive element, electrical irreversible chemical switches fail upon selective reaction of a conductive or resistive material with a chemical species; with failure being indicated by an increase or decrease in resistivity of the conductive or resistive material. The reaction between the conductive or resistive material and the chemical species causes an irreversible phase change in the conductive or resistive material as the chemical species to be detected forms a new phase, such as an alloy, amalgam or a corrosion product. The phase change creates a region or zone, propagated through the bulk of the chemical switch, which causes an abrupt change in the electrical resistance, electrical conductivity or other properties of the switch material. Optimally, the rate of change in electrical or other properties is rapid and has a sufficient magnitude to provide a reliable and measurable indication of the presence of specific chemical components.
An important aspect of the invention is the incorporation of highly durable materials as a selectively reactive element for the conductive or resistive material. These materials should be capable of being engineered in thin or thick-film form as the switch materials. For example, noble metals, either in substantially pure form, or as alloys with other noble metals, are very robust, being highly chemically inert, yet can be engineered for specific chemical reactivity. It is known from the prior art, however, that some noble metals undergo reversible surface reactions with certain chemicals when heated. Noble metal thin films (films of less than approximately 10,000 .ANG. thickness), operating on the basis of surface reactions, have been used as chemical sensors for a limited number of chemical species. Another large class of chemical sensors, metal oxide semiconductor materials, are typically heated to between 300.degree. C. and 1000.degree. C. to facilitate adsorption and desorption of the chemical species on the semiconductor material. Changes in resistivity of the semiconductor material are measured to determine the presence or absence of the chemical species.
Gold thin films have been used to detect the presence of mercury vapor. McNerney, J. J., et al., Mercury Detection by Means of Thin Gold Films, Science 178:611-612 (1972) disclosed detection of mercury vapor by linear changes in resistivity in gold thin films having thicknesses of 75 .ANG. and 400 .ANG., with sheet resistivities of 2 to 10 ohms, respectively. McNerney, et al. suggest that the effects of adsorbed mercury atoms on the conductivity of gold films is a surface effect rather than a bulk alloy effect. U.S. Pat. No. 3,714,562 to McNerney, issued in 1973, (hereinafter "McNerney '562") disclosed that thin film gold layers, having a film thickness of between 75 and 1,000 .ANG., preferably between 75 and 300 .ANG., undergo resistivity changes upon exposure to mercury vapor. The patent contemplates that other thin film metals may be used to detect the presence of other chemicals to which the metal has a chemical affinity. For example, the patent teaches that silver may be used to detect iodine.
The McNerney '562 patent further teaches that if thicker metal films are used, the resistance change becomes masked by the properties of the bulk material. It is taught that the thin films referred to have a mean free path for electrons which is significantly reduced when a contaminant chemical species is adsorbed onto the film. It was found that upon exposure to mercury vapor, the gold thin film exhibited immediate increases in resistance. Over time, the rate of change in resistance increased slowly, which was believed due to amalgamation of mercury with the gold; a reaction that can be reversed by heating the gold.
The sensor described in the McNerney '562 patent consists of a glass plate substrate on which a thin layer of gold has been deposited. The gold layer is configured to provide a reasonably large surface area of gold and a reasonably long resistance path between electrical terminals.
McNerney '562 recognized that the resistance change in the molecular thickness thin film is a function of concentration of the vapors adsorbed. The change in resistivity is due to adsorption of the chemical onto the metal layer. Because the chemical is adsorbed onto the metal layer, without reaction between the gold metal and the chemical, the adsorption is reversible by heating. (Col. 5, L. 13-15). The reversibility of the adsorption is a key difference between McNerney '562 and the present invention, which provides for an irreversible reaction between the switch material and the chemical to be detected. The present invention is also distinguished by the use of switch materials which can cause resistance to decrease, rather than increase, upon exposure to specific chemical components.
Justi, et al., U.S. Pat. No. 3,973,192, disclosed a device for providing an early detection of aerosol products of combustion originating at least partly from a polyvinyl chloride substance. The method consists of measuring the electrical resistance of a thin magnesium foil arranged to be exposed to the aerosol products of combustion. The magnesium foil is provided with a relatively deep corrosion layer of magnesium dichloride to accelerate the corrosion of the magnesium foil upon exposure to the aerosol products of combustion. This method is based on the aerosol products of combustion, which are formed on heating polyvinyl chloride substances to above 100.degree. C. in a moist stream containing hydrochloric acid that can rapidly corrode the magnesium foil. Although this reaction is irreversible, magnesium is a rather reactive alkali earth metal, not a noble metal, and, therefore, cannot reactive selectively or function as a selective sensor. For example, magnesium will readily react with steam to form flammable hydrogen.
Takahama, et al., U.S. Pat. No. 4,224,280, disclosed a device for detecting carbon monoxide which exhibits a stepwise change in film current over a pre-selected range. The Takahama et al. device employs a plurality of semiconductor films. Three embodiments of the device are disclosed. A first embodiment consists of a stannic oxide (SnO.sub.2) film formed on an insulating layer, with a second film layer of predominately platinum (Pt) formed on the first layer of stannic oxide. A second embodiment is identical, except that gold (Au) is added into the platinum layer in a gold-platinum alloy. The second layer is deposited with an average film thickness of 0.3 to 30 platinum atom layers, and the amount of gold ranges to 50 atomic percent of the platinum. A third embodiment contemplates that an electron donor of either antimony (Sb) or bismuth (Bi) is added to the first film layer, and an intermediate layer of stannic oxide having an electron acceptor selected from platinum, aluminum and boron is formed between the first and second films. The insulating film is silicon oxide (siO.sub.2). Electrodes connected to lead wires are used to provide a current in the device. A stepwise change in current results from exposure of the device to an atmosphere containing carbon monoxide. In this invention, a film of platinum or platinum and gold, having an atomic thickness that is narrow enough such that the film does not show a metallic, electrical conductivity, is formed on a film which essentially contains stannic oxide. (Col. 10, lines 31-36). Col. 6, lines 54-65 suggest that use of a gold second layer, i.e., one which is 100% gold, did not yield the characteristic stepwise current change. The use of gold as the second layer is, therefore, not suggested by the reference. Furthermore, FIGS. 7A, 7B and 8 of this patent show that the sensors do not show a very large response to carbon monoxide and that this response strongly depends on the operating temperatures of the device, which must be above 150.degree. C. The inventors admit not to understanding the theoretical basis for the operation of this device. (Col. 6, lines 60-65). Although the inventors do not comment explicitly on the reversibility of their sensors, the response of heated stannic oxide devices is normally reversible.
U.S. Pat. No. 4,587,104 to Yannopoulos disclosed a gas combustible gas sensor which consists of an n-type semiconductor element. The semiconductor oxide is bismuth molybdate Bi.sub.2 O.sub.3 .multidot.3MoO.sub.3. Detection of the combustible gases is based upon the change of electrical conductivity of a thick film of the semiconductor oxide. The semiconductor sensor does not require a catalyst. The express teaching of the of the Yannopoulos patent is that semiconductor sensors are feasible without the presence of a noble metal catalyst, such as platinum, palladium and rhodium. The presence of a catalyst was previously necessary to yield conductivity changes in semiconductor oxide films which were large enough to measure. This reference suggests that it is not necessary, or even desirable, to employ a noble metal element in a gas detector device. The Yannopoulos patent is also based upon the reversible response of this sensor to hydrogen and carbon monoxide.
Komatsu, et al., U.S. Pat. No. 4,592,967, disclosed a gas sensor using mixed oxides, namely tin oxide, at least one lanthanide oxide, and at least one of the IVa group element oxides, e.g., titanium (Ti), zirconium (Zr), hafnium (Hf) or Thorium (Th) in a sintered piece covered with a porous layer of ceramic. The IVa oxide is present in the range of 0.01-20 mol % to keep electric conductance. The gas sensor is constantly heated to 300-450.degree. C. to enable rapid adsorption and desorption of the sensed gas on the sintered semiconductor. This type of device would clearly be unsuitable for applications where operation at ambient temperature is required and it is not based upon irreversible reactions of the sensor material.
Yoshioka, et al., U.S. Pat. No. 4,839,767, describes a device for detecting internal faults in an insulating gas-charged electrical apparatus. The device consists generally of a substrate, a pair of electrodes on the substrate and a thin metal film covering the electrodes and exposed to the substrate surface. The film produces fluorides with low conductivity upon reaction with a decomposed gas produced by internal faults of the electrical apparatus. The patent discloses that the film may be made of silver deposited on a substrate of Al.sub.2 O.sub.3, with gold electrodes. The device is used to detect faults in apparatus charged with SF.sub.6. SF.sub.6 gas escaping through a fault decomposes to SF.sub.4 or SOF.sub.2, which produces HF upon reaction with trace moisture contained in the SF.sub.6 gas. The silver film reacts with the HF to produce AgF which increases the resistance of the silver film. The patent discloses that an order-of-magnitude change in resistance occurs over many hours with a thin Ag film having thicknesses of between 100.ANG.-1000 .ANG.. In one instance, there was a very rapid change in resistance, which required heating of the detection element to 80.degree. C. The need for heating the detection element to obtain sufficiently rapid increases in resistance renders this arrangement unsuitable for a positive identification of chemical species in an ambient environment. In addition, the method disclosed by this patent is restricted to use in special environments, since silver passivates in the presence of oxygen and is not very selective in its reactivity.
The Koda, et al. patent, U.S. Pat. No. 4,938,928, disclosed a gas sensor designed for use at elevated temperatures, e.g., 300-400.degree. C. The device consists of a semiconductor material selected to be specific for the gas to be detected. For example, metal oxide semiconductors of SnO.sub.2, In.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 are used to detect combustible and toxic gases; BaSnO.sub.3, LaNiO.sub.3 and NiO are used to detect oxygen; and ceramics such as MgCr.sub.2 O.sub.3 or TiO.sub.2 are useful for detecting humidity. Noble metals are used for the heat generating members to heat the semiconductor material to facilitate adsorption and desorption of chemical species onto the semiconductor material, resulting in fluctuations in resistance characteristics of the semiconductor. Again, this type of device is unsuitable for applications where operation at ambient temperature is required, and it is not based on irreversible reactions of the sensor material.
Bell, et al., U.S. Pat. No. 5,010,021, and its related U.S. Pat. No. 5,087,574, disclosed a method for detecting a fluid component within a fluid mixture. The method entails the selective adsorption of the component onto a conductive thin layer of material having a chemical affinity for the component, and observing the resulting change of electrical resistivity of the layer. The adsorption is reversible by heating to desorb the chemical species from the thin layer. The patents disclose the use of ozone to increase the dynamic range of the sensor.
With the exception of McNerney '562, Yoshioka '767 and Bell '021, the prior art references teach the use of non-noble metals and/or oxides as the conductive fuse element where the metal oxide's conductance changes measurably upon the adsorption of the particular chemical species to be detected. Typically, the surface adsorption and desorption reactions between the metal oxide and the chemical species occurs at elevated temperatures in the range of 300.degree. C.-1000.degree. C. The ability of the semiconductor material to desorb the chemical species is critical to the various functions of almost all of the prior art detection devices.
McNerney '562 teaches the use of a thin film of gold as a conductive element which becomes resistive upon exposure to mercury vapor. McNerney '562 expressly teaches that there is no reaction between the gold and the mercury vapor, rather the change in resistivity of the gold layer is due to an amalgamation of the mercury, which sequesters gold layer electrons resulting in their unavailability for electrical conduction. While the use of thin layer gold to detect mercury vapor is disclosed, the device and manner of use of the McNerney '562 device is distinct from that of the present invention. Specifically, the McNerney '562 device is designed to detect and measure minute traces of selected chemicals, employing a metal non-reactive with the chemical species. The inert property of gold is utilized to assure that the adsorbed mercury vapor is capable of desorption upon heating of the detection device. As a consequence, the device does not act as an irreversible chemical sensor because the reaction is reversible.
A key difference between the present invention and the detection devices known in the art lies in the selective, irreversible bulk reaction of a chemical species with the chemical switch material in the present device. With the exception of Justi '192 and Yoshioka '767, the prior art discloses reversible surface reactions which induce electrical failure, but the present invention utilizes chemical activity within the bulk of the switch member to induce irreversible changes in the material properties. It is important, in the functioning of a switch, that the change be irreversible. Thus, the reversibility of the adsorption reaction at the surface of the sensors of the prior art sensors renders such devices inherently unsuitable for use as irreversible chemical switches. Furthermore, the nonselectivity, special atmospheres and operating conditions required for effective operation of the Justi '192 and Yoshioka '767 devices render them unsuitable for use as irreversible chemical switches to detect specific chemical components in the environment.
The device of the present invention is capable of undergoing a bulk reaction of the target chemical species at ambient temperature, as opposed to elevated temperatures required by prior art. This is advantageous because it has been found desirable to detect chemical species within the environment in which they may exist. For example, if chlorine gas should leak from a train tank car, a chlorine gas detector must be capable of operation within the temperature ranges in which the train tank car operates.
These devices have the potential to significantly advance emission detection and process control. For instance, in the emission detection arena, they have the unique advantages of being able to pinpoint leaks because they can be made sufficiently small to be placed at the source of the leak. Additional significant advantages of the inventive devices include low cost, low maintenance and no calibration. These chemical switches can find applications ranging from detecting leaks in individual tanks to tank farms to entire manufacturing facilities. Important chemical species with the potential to be detected include hazardous halogens, ammonia, hydrogen chloride, hydrogen fluoride, hydrogen sulfide and methane.